1. Field of the Invention
The present invention generally relates to implantable medical devices, particularly cardiac electrical stimulus generators. More particularly, the invention relates to methods of making hot can stimulation electrodes having a conforming electrically insulative biocompatible coating on the housing. The invention relates still more particularly to methods of selectively removing polymeric coating material from such devices to expose a conductive area of the housing.
2. Description of the Related Art
Biocompatible coatings for implantable medical devices are widely employed to avoid adverse body responses to the implanted foreign object. In many modern implantable heart stimulus generators, a biocompatible coating that adheres to and conforms closely about the generator housing (hereinafter xe2x80x9ca conformal coatingxe2x80x9d) also serves as an electrically insulative layer covering the generator housing.
In the infancy of implantable cardiac pacemaker therapy, the typical pulse generator case or housing was composed of uncoated metal such as stainless steel, titanium or alloys thereof, and was therefore completely electrically conductive. While this permitted the generator case, or housing, to serve as one of the stimulus electrodes, it also allowed an antenna effect between the generator case and the endocardial electrode tip to occur. When flexation of the pectoral muscle occurred, voltages of similar amplitude and frequency to the intrinsic cardiac signals were produced and conducted through the generator case. Spurious undesirable and potentially dangerous influences on the pacemaker functions resulted, the effects including improper inhibition, and in the dual chamber pacemakers, improper triggering and initialization of reentry tachycardias. In addition to the antenna effects on the pacemaker, each time the pacer emitted an electrical pulse to stimulate the myocardial tissue it also stimulated the patient""s pectoral muscle, producing an annoying twitch.
The next generation of pulse generators avoided the muscle-produced improper triggering of the unit by coating the entire electrically conductive case with an electrically insulating material, except for a small uncoated window that allowed the exposed housing surface to serve as the anodal electrode contact. A coating material commonly used for this purpose on pulse generators is typically a thermoplastic polymer film known commercially as parylene, which is both biocompatible and an excellent electrical insulator. In a typical parylene coated unit, the posterior side of the case, meaning all parts facing the (inside) pectoral muscle, all side walls, and part of the anterior (frontal) side of the case are coated with parylene, leaving only a small part of the anterior side of the case to form a forward-facing anodal window that faces the (outside) fatty tissue. This orientation of a xe2x80x9cface windowxe2x80x9d type pulse generator unit greatly reduces muscle-induced interference and has been clinically proven and implemented in many thousands of implantable pacemakers and defibrillators.
Recently, Sulzer Intermedics Inc. provided in U.S. Pat. No. 5,480,416 a cardiac stimulator having its anterior (front) and posterior (back) sides of the case coated with an electrically insulative material such as parylene, but having the edge connecting those two sides at least partially uncoated. Thus, the edge or narrow side of the case functions as the electrically conductive anodal contact surface. This conductive xe2x80x9cedge band windowxe2x80x9d configuration allows for universal implantation orientation of the unit, permitting the pulse generator to be implanted in either the xe2x80x9cnormalxe2x80x9d position conventionally dictated by the placement of the outlets for the connector and lead, or just as conveniently, in the reverse orientation on the opposite side of the body. In this way, the generator case can be turned for left exit or right exit of the electrode lead, and can be implanted in the left or the right side of the patient""s chest, at the option of the physician. This edge band window approach also avoids the muscle-induced interference and inappropriate muscle stimulation problems encountered with prior xe2x80x9chotxe2x80x9d or xe2x80x9cactive canxe2x80x9d electrodes.
Sulzer Intermedics, Inc. has also developed an implantable defibrillator with the conformal coating partially removed to expose conductive windows (U.S. Pat. No. 5,529,579, issued to Alt et al.).
The window in a conventional parylene coated face window pulse generator is typically formed by an oxygen plasma etch process. The plasma etching process usually employs an aluminum fixture or stencil to define the shape and location of the parylene window on the face of the can. In this process, oxygen is ionized by an RF power supply generated electric field. The ionized oxygen gas cloud, or plasma, reacts with the organic parylene in the window region and forms carbon dioxide gas and water vapor. These gases are subsequently removed by a vacuum. The etch rate is controlled by the RF power and the heat generated at that power. High frequency radio frequency power creates a more reactive plasma than does low frequency power. Higher temperature creates a more reactive plasma, as well. The inherent limitations of the battery inside the pulse generator require that the temperature of the entire unit remain below 60xc2x0 C. during the etching process, however. This consideration severely limits the rate at which cans can be passed through the window-etching process. Also, it is necessary to maintain electrostatic shielding to prevent undesirable plasma reactions with the vacuum chamber internal surfaces.
Alternatively, the window can be formed simultaneously with the application of the parylene coating, using a masking technique. For the purposes of a human clinical study, for example, Sulzer Intermedics Inc.""s Edge Band units were manufactured by masking the anodal edge region with weld shield tape, parylene coating the entire unit, then removing the tape. This procedure is time consuming, however, and not feasible for large scale production.
U.S. Pat. No. 5,562,715, issued to Czura et al., describes a silicone rubber or parylene coated pacemaker having detachable tabs that are removed at the time of implantation to expose an electrode. One problem with pacemakers employing pull off tabs is that the resulting window in the coating material has jagged or rough edges that leave the remaining coating vulnerable to tearing, peeling or flaking off into the patient""s body. A rough-edged parylene coating is especially prone to peeling, tearing or flaking while the unit is being manipulated during surgical implantation and thus allows dislodgment of flecks of the coating into the patient""s body. Parylene C is an organic polymer based on p-xylylene, 
having a molecular weight of about 500,000 daltons. It is used as a thin film coating for such diverse applications as microelectronics, digital display systems, medical devices, dry film lubricants and reflectors for optical devices.6 Some of the methods that have been used or examined for removing parylene from various surfaces include heating, mechanical/air abrasion, plasma etching, and excimer laser ablation.1,4 
For cardiac stimulus generators, however, methods that require heating are not suitable because the melt temperature of most polymers is far greater than the maximum 60xc2x0 C. exposure limit imposed by pacemaker battery manufacturers. Mechanical or air abrasion can create static discharges and make edge and face definition virtually impossible. Plasma etching is too slow, requiring over 10 hours to remove the coating layer from the face of a single pulse generator unit. Because the rate of plasma etching is dependent on temperature, achieving faster results would require exposing the unit to temperatures much higher than the internal components can tolerate. Nevertheless, plasma etching remains the prevailing standard method in the pacemaker industry.
Parylene, and other similar biocompatible coatings, also find widespread application in coating medical devices other than implantable electrical pulse generators. Some of these devices are sensors, probes, transducers, stimulators and prostheses.7 In many of these devices it may be desirable to provide a window or uncoated portion similar to the pacemaker window described above.
Recently, coatings for medical articles that incorporate functionally active biomolecules capable of eliciting a particular desired effect in the body have been described. For example, U.S. Pat. No. 5,607,475, assigned to Medtronic, Inc., discusses attaching such biofunctional molecules as anticoagulants, thrombolytic agents, cell attachment proteins and anti-inflammatories to coated surfaces of medical devices by way of a covalent bond. It is anticipated that portions of these coatings will likewise need to be removed in some instances.
Hence what is needed is a more efficient way to manufacture high quality implantable medical devices having biocompatible or biofunctional coatings. In particular, a precision method for selectively removing such coatings from certain areas of the device or article is desired.
The present invention provides a rapid and highly effective process for forming a window in a polymeric coating on an implantable medical device such as a cardiac pacemaker or defibrillator. The present process uses an excimer laser to ablate the adherent thin film coating from the underlying UV-resistant surface of the device. The method is particularly applicable to removal of conformal electrically insulative organic polymer film materials, such as parylene, from the metal surface of a xe2x80x9chot canxe2x80x9d cardiac electrical stimulus generator. The method of the present invention includes subjecting the coated device to an excimer laser beam for a sufficient time to remove a desired thickness of the coat, or to entirely expose the underlying metal surface inside a window of predetermined size and shape. Preferably the metal surface is UV-resistant, or at least has low susceptibility to erosion by excimer laser. The method also provides for defining a precise configuration of the film to be removed by placing a UV-resistant mask or stencil between the device and the laser beam, in order to make a shaped or patterned window in the coating. A stimulus generator housing prepared by the new method has more sharply defined window edges and a more highly polished exposed metal surface than is typically obtained using conventional plasma etching techniques, and coincidentially provides a more esthetically pleasing appearance than conventional hot cans.
The present invention also provides an improved implantable medical device having a biocompatible organic polymer coating with a window exposing an underlying surface, the improvement including making the opening or window in the coating according to the new method. The edges of the window of the improved device have sharply defined edges that resist peeling or flaking of the coating material from the device. Preferably, the implantable medical device is a xe2x80x9chot canxe2x80x9d cardiac stimulation generator and the window serves as an anodal electrode, but it may also be a prosthesis or other implantable medical article having a biocompatible organic coating over a UV-resistant surface and a window or regions of varying thickness in the coating. In some embodiments the biocompatible organic coating is polymeric and comprises a biofunctional molecule.
One embodiment of the invention provides an improved method of making an implantable medical device, or a part of a medical device, that has a window, or ablated area, in an insulative conformal coating that overlies a surface that is substantially resistant to excimer laser erosion. The improved method includes subjecting the device or part to an excimer laser beam for a sufficient time to expose an area of the underlying surface. This new method is especially suitable for use on cardiac pacemakers and defibrillators, particularly those having titanium cases.
Another embodiment of the invention provides a method of removing conformal insulative material from an implantable medical device or part that includes exposing the device or part to an excimer laser beam for a sufficient time to expose an underlying surface that is resistant to erosion by said laser beam. In certain embodiments, the excimer laser susceptible conformal material is electrically insulative, and may contain an organic dielectric in the form of a polymer such as parylene. In some embodiments of the method, a UV resistant mask is interposed between the laser beam and the device or part.
Another embodiment of the invention provides a method of removing an organic coating material from a surface of a medical device or a precursor or component part thereof (xe2x80x9cpartxe2x80x9d). Preferably the surface is resistant to erosion by excimer laser radiation. This embodiment includes providing a source of ultraviolet excimer laser radiation capable of directing a laser beam onto an object at a predetermined point in space; loading the part into a fixture specific to the profile of the part; initiating a computerized controller program; initiating a vacuum hold-down for the part; raising the part pneumatically to a predetermined height fixed by a mechanical stop; initiating a computerized motion program; independently moving X, Y, and rotary axes of said fixture to present an area on the part to a fixed point in space; adjusting the excimer laser source, rep rate, demagnification, number of pulses and etch time, and varying the speed of motion control and distance from the beam to the part, such that a desired level of ablation of the organic coating is achieved; and firing the excimer laser for a sufficient time to permit the irradiated area on the part to receive the desired amount of UV radiation. In certain alternative embodiments, this method is modified to include interposing a UV-resistant mask between the laser radiation source and the part, or moving the part along its X, Y and/or rotary axes while firing the excimer laser.
Still another embodiment of the invention provides an improved implantable medical device, such as a cardiac electrical stimulus generator, or a precursor or component part thereof. In this embodiment, the stimulus generator is of the type having a window in a biocompatible conformal coating, with the window exposing an underlying surface, which may serve as an electrode. Prepared in accordance with the above-described method, the improved stimulus generator has a more highly defined window than that of conventional stimulus generators. This high-definition window may be a xe2x80x9cface windowxe2x80x9d or an xe2x80x9cedge band window.xe2x80x9d